[jnowakow]

What/Why?

Third of four stacked PRs for the shared C++ input core (#1127). Based on the web PR; review only the commits above it.

The iOS input now imports and exports markdown through the C++ core, so it produces the same markdown as web.

• ENRMInputParser.mm and ENRMMarkdownSerializer.mm are thin wrappers over Markdown::Input::parse and serialize.
• cpp/input is added to EnrichedMarkdownCore.podspec and ReactNativeEnrichedMarkdown.podspec.
• The import completion pass (ENRMInputRemend) is removed, so an unclosed *, _ or [ stays literal text.
• The block handlers lose markdownLinePrefixForBlockRange:; the core writes the block markers.
• Selection copy keeps each clipped list item's number.
• docs-md/BREAKING_CHANGES.md lists what changes in stored markdown: escaping, link destinations, nested list indent, delimiter order, and unsupported syntax (quotes, code, tables) losing its markers on import.

Testing

PR Checklist

• Code compiles and runs on iOS
• Code compiles and runs on Android
• Updated documentation/README if applicable
• Ran example app to verify changes
• E2E tests are passing
• Required E2E tests have been added (if applicable)